The present invention relates to the field of percutaneous transluminal angioplasty generally, and more particularly to a stent delivery system for producing variable post-deployment stiffness characteristics in stents which have uniform pre-deployment radial stiffness.
The use of balloon catheters for high pressure dilation of occluded blood vessels is well known. Balloon coronary angioplasty, for example, is often used as an alternative to open-heart coronary bypass surgery. In a typical balloon angioplasty procedure, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the femoral arteries by means of a conventional Seldinger technique and advanced within a patient's vascular system until the distal end of the guiding catheter is positioned at a point proximal to the lesion site. A guidewire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guidewire sliding within the dilatation catheter. The guidewire is first advanced out of the guiding catheter into the patient's vasculature and is directed across the arterial lesion. The dilatation catheter is subsequently advanced over the previously advanced guidewire until the dilatation balloon is properly positioned across the lesion. Once in position, the expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressures, usually in the range of about 6-12 atmospheres. Balloon expansion radially compresses the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilates the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter may be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery. As should be appreciated by those skilled in the art, while the above-described procedure is typical, it is not the only method used in angioplasty.
Balloon angioplasty sometimes results in short or long term failure. That is, vessels may abruptly close shortly after the procedure or gradual restenosis may occur up to several months afterward. To counter the tendency of recurrent vessel occlusion following angioplasty, implantable intravascular prostheses, commonly referred to as stents, have emerged as a means by which to achieve long term vessel patency. Stated simply, a stent functions as permanent scaffolding to structurally support the vessel wall and thereby maintain luminal patency. Stents are typically small tubular metallic structures.
Since the present invention is directed to an improved stent delivery system, it may prove useful to briefly describe the components and operation of a typical stent delivery system. Such systems typically include a balloon catheter, a stent which is mounted on the balloon, and a delivery sheath which surrounds the stent-delivery catheter. Initial angioplastic dilation of the lesion produces a residual lumen large enough to accept the stent delivery system. The guiding catheter used to perform the initial dilation is typically left in place in the patient and reused during the stent implantation procedure. The stent-delivery catheter is routed through the guiding catheter to a position in which its distal end is disposed substantially coextensively with the distal end of the guiding catheter and immediately proximate of previously expanded lesion.
Once properly positioned relative to the guiding catheter, the stent-carrying catheter is extended from the distal end of the guiding catheter until the stent spans the previously dilated lesion. The delivery sheath which is slidable relative to the delivery catheter, balloon and stent, is then withdrawn into the guiding catheter to expose the balloon and stent. The delivery catheter is then supplied with a pressurized fluid, which expands the balloon and associated stent to a desired diameter sufficient to exceed the elastic limit of the stent. The stent thus comes in contact with, and permanently supports, the wall of the vessel. The delivery catheter balloon is then deflated and the delivery catheter and guiding catheter are withdrawn, leaving the expanded stent supporting the vessel lumen.
Prior art stent delivery systems have generally proven to be effective. However, in the treatment of certain vascular diseases, it is desirable to have a stent with reinforced end rings or regions of relatively high stiffness at one or both ends of the stent. Such a stent is required to successfully treat ostial vessel diseases such as in the renal vessels. The ostium of the renal vessels requires a stent with an end region which possesses relatively high resistence to radial compression. This is due to the close proximity of the aortic wall muscles which have a tendency to contract around the renal ostium and which may cause radial collapse of a dilated vessel implanted with a non-reinforced stent. Vessels such as the coronary sinus, ostial RCA, and ostial left main, and other vessels where the ostium is surrounded by tissue which produces high radial forces, may also be beneficially treated by a stent with a reinforced or high stiffness end region. Stents of this type are commonly referred to as variable stiffness stents. The present invention satisfies these and other needs.